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Mysterious bursts of activity in flatlining brain

By Andy Coghlan

PARTS of the brain may still be alive even when someone is in a deep coma and their brain activity seems to have gone silent.

When a person enters a deep coma an electroencephalogram (EEG) may eventually show a flatline, which is one of the signs of brain death. However, while monitoring a patient who had been placed in a coma to prevent seizures following a cardiac arrest, Bogdan Florea at the Regina Maria Medical Centre in Cluj-Napoca, Romania, noticed something strange – tiny intermittent bursts of activity interrupting a flatline signal, each burst lasting a few seconds.

He asked Florin Amzica of the University of Montreal in Canada and his colleagues to investigate what might be happening. In an attempt to create a similar scenario, Amzica’s team put cats into a deep coma using a high dose of anaesthetic. While EEG recordings taken at the surface of the brain – the cortex – showed a flatline, those from deep-brain electrodes revealed tiny bursts of activity. Those mysterious signals originated in the hippocampus, which is responsible for memory and learning, and spread within minutes to the cortex (PLoS ONE, doi.org/nws).

“These ripples build up a synchrony that rises in a crescendo to reach a threshold where they can spread beyond the hippocampus and trigger activity in the cortex,” Amzica says.

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Amzica speculates that those signals may help keep the brain ticking over when someone is in a coma. But he also thinks that the ripples, which he dubbed nu-complexes, are so tiny that they won’t show up on most EEGs.

Withdrawing the anaesthetic was enough to bring the cats out of their coma. Florea’s patient also revived after coming off anti-seizure drugs, but died a month later from heart complications.

Does this mean that people who have been diagnosed as brain-dead actually have hidden activity that suggests they could recover?

No, says Steven Laureys of the University of Liège in Belgium, because tests for brain death are extensive. They generally combine EEGs showing irreversible and complete cessation of brain activity with tests that measure whether blood is getting to the brain. If there is no blood supply, the brain will die or already be dead.

In some circumstances, blood still reaches the brain when it shows a flatline, during the deep coma Amzica induced in his patient, for example, or if a person is heavily sedated or very cold. It can happen in hibernating animals, too.

The new findings are of great value, says Laureys, offering us a better understanding of brain activity in these circumstances. “We should abandon the idea that a flat EEG proves there is no residual neural activity,” he says. “It’s something to be investigated further because the role of these ripples in coma is ill-understood.”

This article appeared in print under the headline “Mysterious burst of activity in flatliners”